1. Field of the Invention
The present invention relates to an improved dynamic current transducer that can be used in any electrical circuit in which the variations of current with respect to the time are substantial and are to be analyzed even if they are very short, or in any electrical equipment and/or component in which a high power alternating current has to be measured over a very long period of time, or even in a permanent manner.
Therefore, the invention relates to an improved, dynamic current transducer that can be used, for example, as an unsaturable current monitor subject to minimal internal heating, for the development of high power electrical equipment such as high power, high voltage circuit breakers, commutators or lightning arresters. Similarly, the improved, dynamic current transducer according to the invention can be used for replacing a current transformer in a high voltage transmission line.
2. Discussion of Related Art
It is known that the measurement of the amount of post-arc current in a circuit breaker is essential for understanding the various phenomena which are specifically related to the operation of one given breaker. Indeed, it is known that the quality of a circuit breaker depends on the intensity and of the rate of decrease of this post-arc current which is usually small (lower than 10 amps) and has a very sharp rate of decrease (a few microseconds). On the other hand, it is known that the development of a circuit breaker which has a good frequency response, a large output voltage and a heavy duty breaking capability, depends on the measurement of the amount of post-arc current and of the rate of decrease of current at nearly 0 amps of the circuit breaker.
The measure of the dynamic evolution of the small current which follows the interruption of high current when a circuit breaker is tripped gives important information that can be used to determine the efficiency and capabilty of a circuit breaker and thus its quality.
However, it is also known that the measurement of this small current following the interruption of high current in a circuit breaker is very difficult to carry out in practice and has required up to now the use of very sophisticated methods (see, for example, the article of M. Murano et al., IEEE, P.E.S. Winter Meeting, paper NO. T-75-70-8, 1975).
One of these methods makes use of shunts of increasing sensitivity, which are successively switched on when the current decreases towards zero amps, (see G. Hortopan et al., Revue Generale de l'electricite, no. 5, pages 332 to 343, 1981). Another method used for measuring this small current makes use of transformers with saturable cores, such as those manufactured and sold by Pearson Electronics, Inc., or of transformers with Hall effect cores.
If these known methods have some advantages, they all have drawbacks too.
Thus, for example, in the first method mentioned hereinabove, the use of a given number of shunts connected in parallel require operation and synchronization of solid state or electro-mechanical control means, or of one or more additional circuit breakers having a very high frequency response. The use of such additional circuit breakers is required to protect the shunts in the high current phase.
On the other hand, the second method mentioned hereinabove has major drawbacks too. By way of example, the transformers with saturable cores usually have a frequency response which is very small (see the article entitled "Transient response of current transformer" IEEE Power Engineering Society, Power System Relaying Committee, 76-CH-1130-4 TWR, 1976). Similarly, the transformers with Hall effect cores require a very substantial active system and a high power supply.
The other methods known in this field, such as those using high responsive diodes connected in parallel with shunts in order to filter the high signal peaks, are not very convenient and are often inaccurate.